Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of the first image of a display screen or portion thereof with a transitional graphical user interface showing the new design;

FIG. 2 is a front view of the second image thereof;

FIG. 3 is a front view of the third image thereof;

FIG. 4 is a front view of the fourth image thereof;

FIG. 5 is a front view of the fifth image thereof;

FIG. 6 is a front view of the sixth image thereof; and,

FIG. 7 is a front view of the seventh image thereof.

The appearance of the image transitions sequentially between the images shown in FIGS. 1-7. The process or period in which an image transitions to another forms no part of the claimed design. The broken line showing of the display screen or portion thereof, text and graphic elements forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 